Low Altitude Troubles

[[email protected]]

An email correspondent asked me to explain why low-altitude thermaling is apparently so dangerous. After all, he (and I) have never unintentionally spun at high altitude, so why is it so bad to thermal down low? He suggested the answer might be of more general interest so here it is.

---

It's not really "more likely to spin" but surely "more likely to get in to big trouble."

- Thermals form in very small little bubbles and streams down low. These coalesce together between 500 - 1000 feet to form the larger structures we use. Many of the little streams and bubbles die out. The lower 500 feet is (you hope) super-adiabatic which means unstable in both directions. If a little bubble tries, a bigger bubble nearby can push it down and quash it. At any rate, lift down low is much smaller, gustier, rattier and less reliable.

- In the bottom 500 feet you are in the turbulent boundary layer, just like in the last mm of your wing. This means there is lots of wind induced turbulence, lots of little gusts and eddies. Many things that feel like thermals will vanish and turn in to sink and tailwind when you turn. The overall gustiness is greater in general.

- Ground illusions are very strong. Just at the moment when you are really busy you have to deal with a totally different view out of the cockpit. Doing 60 mph heading downwind it is really hard for your brain to realize you're on the edge of a stall.

- Add these up. You're going in to the wind. You feel a gust - which is just a gust, but you don't know it. The glider rises, the vario chirps, you feel g in the pants. Whew! You turn. As you pass 90 degrees you're also getting the "dynamic soaring effect" that the gust is accelerating the glider. But as you hit 180, the gust turns in to lull, which means you're headed downwind and lost 10-20 knots of airspeed, and the nose points down. But since you're headed downwind the trees are going by really fast, you think you have plenty of speed, the nose is pointed down (since you just got dumped), so you pull back on the stick

I don't think the vast majority of accidents resulting from low-altitude thermaling attempts are simple stall-spin entries in the thermal. I think crashes out of thermaling attempts are really sink/gusts. This seems to be for example the consensus of the crash at Ionia -- the gust robbed him of airspeed at the wrong moment, the glider fell out of the sky and just didn't have room to pick up speed.

The stall-spins come from very low altitude maneuvering after the thermaling attempt expires. If you try to thermal at 400 feet, you give up at 300 feet... and 40 knots and pointed the wrong way. Now you have to do a lot of last minute maneuvering to get to the field.

The last minute maneuvering is where the stall/spin happens - lined up wrong, changing fields at the last moment, etc.

Here too the ground illusions set in. You left the thermal doing 40 knots, as you were thermaling. Or, you got dumped on the backside of the gust. Now there will be big downwind illusion on your "downwind" plus a mad desire not to lose altitude -- it's not often you start a pattern at 300 feet and 40 knots. Subconsciously, you will not want to give up another 200 feet to get the airpseed up where it belongs. As you make last minute maneuvers -- let's not call this a pattern -- you are still used to the sight picture of turning on a point that happens high up. That a turn takes radius may come as a surprise. Remember you're under huge stress here. If you don't line up with the field, and you are uncomfortable banking -- now at 100 feet and still 40 knots -- your subconcious will want to use the rudder. And as the nose falls, pull back.. here we go.

In sum, trying to thermal below 500 feet is much less likely to work, because the thermals are much smaller and the wind gusts much worse and more tempting. The biggest danger is that these attempts will lead to a low, tight, slow pattern, and the last minute maneuvering can easily set up the stall spin.

On a cheery note, this sort of thinking sets some safety parameters. It's more dangerous to thermal low if you don't have the landing field exactly mapped out and your approach to it; if the wind is stronger; if the thermal day (super-adiabatic layer) is stronger; if there are trees or hills contributing to low altitude turbulence; and if your stress level and ability to adapt to the different air and ground illusions is weaker.

---

Not a cheery thought for a cold winter day, but a scenario worth rehearsing.. The temptation to keep trying and avoid a landout is oh so strong, and all the ways the air is different down low not so obvious.

John Cochrane

[Bob Whelan[_3_]]

On 1/30/2014 5:09 PM, wrote:
> An email correspondent asked me to explain why low-altitude thermaling is
> apparently so dangerous. After all, he (and I) have never unintentionally
> spun at high altitude, so why is it so bad to thermal down low? He
> suggested the answer might be of more general interest so here it is.
>
> ---
>
> It's not really "more likely to spin" but surely "more likely to get in to
> big trouble."
<Much good stuff snipped...>

> The stall-spins come from very low altitude maneuvering after the
> thermaling attempt expires. If you try to thermal at 400 feet, you give up
> at 300 feet... and 40 knots and pointed the wrong way. Now you have to do a
> lot of last minute maneuvering to get to the field.
<More snippage...>

> The biggest danger is that these attempts will lead to a
> low, tight, slow pattern, and the last minute maneuvering can easily set up
> the stall spin.

John's assessment concludes (and I agree with him) that low thermalling
attempts can easily lead Joe Pilot to enter what I call a "drunken sailor"
landing pattern. You know...staggering around erratically while J.P. figures
out what to do, when.

Be aware this tendency isn't limited only to unsuccessful attempts to thermal
away from down low. Over the years, I've seen it with some regularity at club
camps (when pilots of various skills fly from a new-to-them field) and when
winching (ditto the camp/unfamiliar field scenario, plus perhaps, patterns
entered from an unusual point (e.g. directly above the intended landing
runway, from a lower than home-field-typical height after a poor launch or
premature release or whatever...).

The drunken sailor pattern isn't limited to lower time pilots, either, in my
observation.

One of the funnier (to my dry sense of humor, anyway) instances occurred at a
winch camp (no other power or glider traffic; single paved runway;
calm/unsoarable conditions) when a multi-hundred-hour, XC-experienced, pilot
in a nose-hook-only 2-33 released above the winch at perhaps 600' agl. Under
the circumstances, it shoulda been a no-brainer to simply ease over into a
downwind via a single 180, and land short for the next snap after the 2nd
180...with maybe some practice maneuvering thrown in just because he could and
conditions (no other traffic) safely permitted. Instead, a series of
altitude-devouring, genuinely puzzling - pointless, to inquiring minds, when
the fact a downwind to the paved runway would eventually be required (no
off-field options within reach) - wanderings and meanderings ensued, to the
point I began to worry we might be about to witness a crunch. Happily, the
eventual arrival was crunch-free...but the "actual pattern" was dubiously
worrisome to me, so much so that I engaged the pilot (a soaring friend) in
post-flight conversation, the gist of which was, "What were you THINKing (and
doing)?!?" What was funny about it was his explanation matched what my mind's
eye had seen...it truly HAD BEEN a drunken sailor pattern, and he truly had
not had any plan he was attempting to execute between the release and the
(eventual) moment he realized he needed to "do something" if he was going to
land safely. It was funny only because we both could laugh about
it...lesson(s) learned for him, I figured. In any event I never again saw him
fly with his brain so obviously not fully engaged.

But being my club's newsletter editor, I was subsequently surprised to find
Joe Offending Pilot was offended (happily, only in the short term) to see that
situation described & discussed in the next month's newsletter's safety
article, even though it was done in a manner anonymizing the culprit's
identity. I think he was offended because he was still embarrassed. A year or
so later we were both laughing about it.

So...your future glider arrivals are likely to be safer if you work actively
to avoid the drunken sailor pattern, regardless of actual circumstances, low
thermalling induced or not.

Bob W.

[[email protected]]

By email a followup comment, which struck me the group might want to read too.

..."at normal altitudes I am unconsciously using the horizon to set the pitch and thus the airspeed. When I get really low, the horizon is commanding me to pull the nose up - to put the horizon where it belongs.

In normal flatland flying, we're always in the pattern when we're low and we don't realize how far below the horizon the nose has to be just to maintain airspeed."

This is good point - and another warning to pilots. Yes indeed. The horizon is higher. This is a well known problem for flatland pilots flying in mountains. Also the picture fills with trees which are a lot bigger closer to them. Either way you put it the "sight picture" for proper pitch attitude is way different at 500 feet than 5000 feet.

I've seen a related error while instructing. When students get high on final, they push the nose down so the glider is pointed where they want to go. Of course, and especially in the duo discus, pushing the nose down just makes the glider go faster on the same glide path. The nose controls angle of attack, not glider flight path. So, it stands to reason that the same pilots will pull the nose up to make the glider go on a shallower flight path.

All three effects conspire. You don't want to get lower, so you don't trade altitude for speed. You don't really believe that the glider will go flatter at 53 knots than at 40 knots though the nose is pointed down more. You don't want to point the nose down because the trees fill the sight picture, and the horizon seems too high.

I think pilots don't pay attention because any bozo understands all this on the ground. The trouble is that when you're under a lot of stress, the subconscious takes over and it's oh so easy to forget all this. I once had a student on base try to rudder it only. I asked about bank and he actually said "I'm trying but the controls are stuck." Subconscious aversions are really strong.

Next time there is a strong crosswind, count how many pilots overshoot base to final. It's a good test for "are you thinking ahead about how things are going to be different than what you subconsciously expect?"

John Cochrane

[JJ Sinclair[_2_]]

On Friday, January 31, 2014 6:57:01 AM UTC-8, wrote:
> By email a followup comment, which struck me the group might want to read too.
>
>
>
> .."at normal altitudes I am unconsciously using the horizon to set the pitch and thus the airspeed. When I get really low, the horizon is commanding me to pull the nose up - to put the horizon where it belongs.
>
>
>
> In normal flatland flying, we're always in the pattern when we're low and we don't realize how far below the horizon the nose has to be just to maintain airspeed."
>
>
>
> This is good point - and another warning to pilots. Yes indeed. The horizon is higher. This is a well known problem for flatland pilots flying in mountains. Also the picture fills with trees which are a lot bigger closer to them. Either way you put it the "sight picture" for proper pitch attitude is way different at 500 feet than 5000 feet.
>
>
>
> I've seen a related error while instructing. When students get high on final, they push the nose down so the glider is pointed where they want to go.. Of course, and especially in the duo discus, pushing the nose down just makes the glider go faster on the same glide path. The nose controls angle of attack, not glider flight path. So, it stands to reason that the same pilots will pull the nose up to make the glider go on a shallower flight path..
>
>
>
> All three effects conspire. You don't want to get lower, so you don't trade altitude for speed. You don't really believe that the glider will go flatter at 53 knots than at 40 knots though the nose is pointed down more. You don't want to point the nose down because the trees fill the sight picture, and the horizon seems too high.
>
>
>
> I think pilots don't pay attention because any bozo understands all this on the ground. The trouble is that when you're under a lot of stress, the subconscious takes over and it's oh so easy to forget all this. I once had a student on base try to rudder it only. I asked about bank and he actually said "I'm trying but the controls are stuck." Subconscious aversions are really strong.
>
>
>
> Next time there is a strong crosswind, count how many pilots overshoot base to final. It's a good test for "are you thinking ahead about how things are going to be different than what you subconsciously expect?"
>
>
>
> John Cochrane

Forty years ago, I found myself landing out and down-wind! The (your going too fast illusion) was almost irresistible. I think the only thing that saved me was I could feel the ship was going as slow as it could.

We had a tragic accident out here several years ago that was caused by the down-wind illusion. The non-current pilot took a late afternoon wave tow into a 30 knot wind. He was unable to stay in position due to rotor turbulence............when the rope broke, he was in the correct position to fly a down-wind leg and land back. Flying about 60 knots + the 30 knot tail wind, resulted in a 90 knot down-wind leg. He just knew the sagebrush shouldn't be going by that fast, so he slowed the ship down.............stalled............spun.......... .and is no longer with us!

Good paper, John
JJ

[Sean F (F2)]

I fully agree with the comments and conclusions. Thermalling low (especially in significant wind) is enormously risky. Anyone who says it is not is part of the problem. It is far more risky if you are subconsciously incentivized to resist landing out via competition. I have saved in a contest once from 225 ft. My first time flying away from Ionia actually. Completely stupid.

Thermalling low is like being above 26,000 ft. on Mt. Everest. "The death zone" as they call it in the mountain climbing world. In gliding, you know once you get down below 1000 ft. AGL that you are putting yourself into considerable danger. Below 600 that danger is now extreme, especially when windy. Every time you do this you are accepting a much higher likelihood that a mistake will result in a crunch. There is little chance of spin recovery here.

After Tim's Ionia accident & Derrick Mackie's (Canada) accident I realized that I was very capable of making the same mistakes they did. I knew Tim and knew Derrick very, very well. I was very, very nervous about the sport for awhile. I eventually set a limit of 750 AGL. Below that level I will land, period. I have been very disciplined about it so far. This has resulted in many more landouts than before but a feeling of inner peace after I am on the ground (safe and controlled manner) in that I have respected my limit and mitigated the risk.

I also have a wind limit that I consider in regards to launching. For example, one day at Sports Class Nationals in Parowon there were winds of 25-35+ basically coming almost right down the runway. Large mountains run alongside the airport. Gusts were huge! Porta potties blew over and downwind at the end of the runway and crews being forced with hold the gliders down. Wings were shaking, etc. Dust was limiting visibility, etc. The idea of launching was debated. Eventually, I simply pulled my glider to the side and waited until the grid was gone and went back to the trailer. 2 or 3 others did the same including some highly experienced contest pilots. During the "debate" phase, the soaring "players" were upset that a previous high wind day had been "called" for safety although a pilot later took a launch and was able to get up into the higher altitudes where conditions were solid.. He flew the task and the players were upset that a day had been lost. This next day, the one where I pulled off, the winds were actually stronger than the cancelled day... Great pressure was put on the CD to go... Something else to think about perhaps. I found this whole subjective process to be highly questionable in terms of making the decision to fly.

Sean

[[email protected]]

On Friday, January 31, 2014 7:52:24 AM UTC-8, JJ Sinclair wrote:
> On Friday, January 31, 2014 6:57:01 AM UTC-8, wrote:

Additional compounding illusion that most of us know from theory and instruction but may forget when flying instinctively under stress:

Above a certain altitude when you make a coordinated turn with a given bank angle the inside wingtip traces a circle in the opposite direction vs the ground. In a low turn to final the wingtip goes the same way around versus the ground leaving a subtle sensation that the turn lacks enough rudder.

So you are too slow from a high horizon and pulling the nose up to the back side of the polar in a misguided effort to stretch your glide, while at the same time you have a tendency to under-bank to keep the wing away from the ground and over-rudder due to the wingtip motion illusion.

All these illusions nudge you in the wrong direction at the wrong time.

On a related topic - I installed a new flight computer/vario last year (an LX 9000). Like a lot of current generation instruments it has accelerometers built into it. I believe the primary purpose for these is to add some additional capability to separate horizontal gusts from vertical lift (that surge in the seat of your pants that we all know and love). They seem to still be working out exactly how to do this in the software.

The other think they appear to be doing is using some combination of airspeed, g-load and pilot input wing loading to estimate angle of attack and provide a stall warning. I've only done a little testing with it but it appears to be pretty reliable.

Given that stall/spin has been the cause of 39% of all fatal glider accidents and 36% of fatalities over the past 20 years a decent warning system might help. Has anyone done any serious testing of the accuracy of these systems under the kind of scenarios in this thread?

9B

[Dan Marotta]

A problem we continue to see at high altitude airports is low altitude
visiting pilots not accounting for the higher true airspeed. As they get
into the flare they feel they're flying too fast (TAS) and slow down too
much (IAS). We see some hard landings.


> wrote in message
...
On Friday, January 31, 2014 7:52:24 AM UTC-8, JJ Sinclair wrote:
> On Friday, January 31, 2014 6:57:01 AM UTC-8, wrote:

Additional compounding illusion that most of us know from theory and
instruction but may forget when flying instinctively under stress:

Above a certain altitude when you make a coordinated turn with a given bank
angle the inside wingtip traces a circle in the opposite direction vs the
ground. In a low turn to final the wingtip goes the same way around versus
the ground leaving a subtle sensation that the turn lacks enough rudder.

So you are too slow from a high horizon and pulling the nose up to the back
side of the polar in a misguided effort to stretch your glide, while at the
same time you have a tendency to under-bank to keep the wing away from the
ground and over-rudder due to the wingtip motion illusion.

All these illusions nudge you in the wrong direction at the wrong time.

On a related topic - I installed a new flight computer/vario last year (an
LX 9000). Like a lot of current generation instruments it has accelerometers
built into it. I believe the primary purpose for these is to add some
additional capability to separate horizontal gusts from vertical lift (that
surge in the seat of your pants that we all know and love). They seem to
still be working out exactly how to do this in the software.

The other think they appear to be doing is using some combination of
airspeed, g-load and pilot input wing loading to estimate angle of attack
and provide a stall warning. I've only done a little testing with it but it
appears to be pretty reliable.

Given that stall/spin has been the cause of 39% of all fatal glider
accidents and 36% of fatalities over the past 20 years a decent warning
system might help. Has anyone done any serious testing of the accuracy of
these systems under the kind of scenarios in this thread?

9B

[BobW]

On 1/31/2014 11:06 AM, Dan Marotta wrote:
> A problem we continue to see at high altitude airports is low altitude
> visiting pilots not accounting for the higher true airspeed. As they get into
> the flare they feel they're flying too fast (TAS) and slow down too much
> (IAS). We see some hard landings.

While we're talking about perceptions (and their power to affect pilot
reactions/realities) and data points...

After several decades of "learning how to soar" in the intermountain west and
having the sight picture(s) of gliders landing at airports ranging in
elevation from 5,000' msl to 8,000+' msl become part of "my normal reality," I
had opportunities - on a trip back to MD (where I obtained my license) - to
visit some gliderports (~800' msl). Even though I knew the physics of things,
and fully expected a different sight picture, from beside runways it looked to
me as if gliders landing "at sea level" were flying through molasses. They
were SO slow throughout final, into the flare and to touchdown! "Suddenly" the
reason GA airstrips at sea level tend to be half the length of those in the
intermountain west became (more) obvious!!! :-)

Regardless of from what elevation one launches and lands, IAS and "glider
feel" clearly should trump the sight picture, *especially* when it comes to
low-altitude flight...

Bob W.

[Ramy]

I think most will agree that thermaling below 500 feet is not a good idea and will seldom work anyway.
But what are your thoughts about thermaling low above ridges or hills? Are the same rules apply? Many of us do it often and usually it is hard to tell your exact altitude above the ridge if you don't know the ridge elevation.

Ramy

[Dan Marotta]

I don't thermal low above ridges often but, when I do, I keep my speed up
and have an exit strategy in my back pocket. Stay alive, my friends...

(With apologies to Dos Equis)


"Ramy" > wrote in message
...
>I think most will agree that thermaling below 500 feet is not a good idea
>and will seldom work anyway.
> But what are your thoughts about thermaling low above ridges or hills? Are
> the same rules apply? Many of us do it often and usually it is hard to
> tell your exact altitude above the ridge if you don't know the ridge
> elevation.
>
> Ramy

[[email protected]]

> But what are your thoughts about thermaling low above ridges or hills? Are the same rules apply? Many of us do it often and usually it is hard to tell your exact altitude above the ridge if you don't know the ridge elevation.

My thoughts are, er, "evolving." Due to a youth mis-spent flying hang gliders and some ridge time, I got used to turning really close to hills. Too many magazine articles rave about rock polishing. However, we have had a spate of crashes lately from pilots -- really good, famous pilots -- running in to hills. Peter Masak, Klaus Holinghaus are just two of the many.

Doing the 2012 safety review analysis of Brian Milner's flight -- available here

http://faculty.chicagobooth.edu/john.cochrane/soaring/index.htm#safety_rules

was another eye opener for me. Brian thermaled off the ridge, found 4 knots up at the 90 degree mark, still climbing at 180 degrees, well above ridge top height and decently away from the ridge -- now, tighten and finish the 360? He did, and at 270, the lift turned to sink, and tailwind, and he hit the top of the ridge. Remember, he was well above the ridge when he made the crucial decision to finish the turn. At least looking at the see you screen shots, it' something I have done many many times with equal or less clearance.

Frank Paynter's soaring cafe post about his crash at Moriarty is another eye opener. Standard ridge technique, and Frank is no rock-polisher, and all it takes is one big gust of wind.

For me, a trip to New Zealand was another eye opener. I expected, based on articles, to be told to get close to the mountains. No, in fact, my instructor G Dale was aghast at how close I was flying and how soon I wanted to thermal. Hmm.

Bottom line. Close to hills lie many of the same traps, and I think there are traps out west for us east-coast ridge pilots.

-Wind will be more turbulent close to the hill, especially if the ground features are more disruptive.

-More sun and unstable air makes it more turbulent still -- you're flying in a super adiabatic layer, made more so by lifting.

-There is not just lift and sink, there is headwind and tail wind. A gust of tail wind when you're pointing at the hill is no good.

-Ground illusions abound. In particular in the air, you think subconsciously that you're turning on a point, and forget how much radius a turn takes.

-If you follow common advice and add some airspeed, your turn radius gets larger still. Turn radius goes with airspeed squared. Adding water ballast really increases your turn radius, because you are flying 5-10 knots faster.. As an ex hang glider pilot, the huge turn radius close to the ground really surprised me on my first few mountain flights.

-By email, I was reminded of another illusion. When you really want to turn, now, you really want to see the nose sweeping across the horizon. Nothing like stomping on the rudder to get that accomplished.

And a bit of Russian Roulette. You can get away with things 99% of the time and, like me, convince yourself you're being safe. The 1% monster out there has gotten so many pilots.

In sum, I'm going to be putting a bit more distance between me and the mountain!

John Cochrane

[SoaringXCellence]

Another idea that has been discussed regarding close ridge thermalling:

There can be both strong updrafts and downdrafts within one wingspan resulting in a force greater than the rolling forces of the ailerons. JJ wrote an article "Don't smack the mountain" about just that concept.

Mike

[Ralph Jones[_3_]]

On Fri, 31 Jan 2014 11:06:55 -0700, "Dan Marotta"
> wrote:
[snip]
>
>Above a certain altitude when you make a coordinated turn with a given bank
>angle the inside wingtip traces a circle in the opposite direction vs the
>ground. In a low turn to final the wingtip goes the same way around versus
>the ground leaving a subtle sensation that the turn lacks enough rudder.
>
[snip]

Pivotal altitude. Equal to TAS in knots, squared, over 11.3, or about
319 AGL at 60 knots.

rj

[Craig R.]

> There can be both strong updrafts and downdrafts within one wingspan resulting in a force greater than the rolling forces of the ailerons. JJ wrote an article "Don't smack the mountain" about just that concept.
>
>
>
> Mike

JJ's article can be read here. It is worth a look.

http://www.dg-flugzeugbau.de/index.php?id=mountain-e

[[email protected]]

On Saturday, February 1, 2014 8:42:15 PM UTC-5, Craig R. wrote:
> > There can be both strong updrafts and downdrafts within one wingspan resulting in a force greater than the rolling forces of the ailerons. JJ wrote an article "Don't smack the mountain" about just that concept.
>
> >
>
> >
>
> >
>
> > Mike
>
>
>
> JJ's article can be read here. It is worth a look.
>
>
>
> http://www.dg-flugzeugbau.de/index.php?id=mountain-e

ThereisanotherexcewllentarticlewrittenbyHarryCombs regardingmountainridgethermalsandthepropensityfors tallsandunexplainedcollisionswithterrain:http://www-ee.stanford.edu/~hellman/soaring/Combs.pdf

[Dan Marotta]

It wasn't me that wrote the quote below, but thanks for the math. BTW,
there's absolutely no way I'd be thermalling at that altitude. I'll be
lining up on a landing spot (or a tree).


"Ralph Jones" > wrote in message
...
> On Fri, 31 Jan 2014 11:06:55 -0700, "Dan Marotta"
> > wrote:
> [snip]
>>
>>Above a certain altitude when you make a coordinated turn with a given
>>bank
>>angle the inside wingtip traces a circle in the opposite direction vs the
>>ground. In a low turn to final the wingtip goes the same way around
>>versus
>>the ground leaving a subtle sensation that the turn lacks enough rudder.
>>
> [snip]
>
> Pivotal altitude. Equal to TAS in knots, squared, over 11.3, or about
> 319 AGL at 60 knots.
>
> rj

[son_of_flubber]

On Saturday, February 1, 2014 8:42:15 PM UTC-5, Craig R. wrote:
> JJ wrote an article "Don't smack the mountain" about just that concept.
> http://www.dg-flugzeugbau.de/index.php?id=mountain-e

Dang. This is interesting. I do not fly in "mountains", but we have ragged and undulating relatively high ridges close-in to the east and west of my home airport and depending on the wind direction they throw rotor, turbulence and gusty lift in front of each other.

JJ says:
>>As I get within 300 feet of the rocks, I roll the ship into a 30 degree right bank and hold it there with a bit of top rudder. Why? Because, I'm already banked away from the mountain, so if a zephyr tries to get me, I'm ready to apply all three control inputs to fight it with right stick, right rudder and back stick to make it turn away from the rocks.<<

If I understand this, JJ is maintaining a constant turn to the right relative to the air mass movement (aka the wind) and thereby maintaining a flight path parallel to the slope, and since he is flying a high performance glider, he does not see much altitude loss due to the slight slip induced by the top rudder.

Having trained in low to medium performance gliders, I learned to crab (wings level) to fly parallel to a ridge. JJ seems to be saying that dropping the upwind wing is a better solution. I can see that this also makes it easier to observe, note and correct any residual drift towards the ridge (whereas the crab directs your attention somewhat away from the ridge).

JJ says:
>>It is estimated that our ailerons can only counteract a differential moment of 2,5 m/s. Let's say there's 6m/s under our right wing and only 1,5 m/s under our left wing. Reasonable figures? Sure, we see 6m/s all the time on the Whites and in the Sierras<<

I've rarely seen sustained 6m/s, but the gusty rotor/turbulence near the local ridge might easily produce a strong pulse. And if my reaction time is a little slow, then lesser differential lift will flip me. I've honed my reflex to keep the wings level when I hit rotor on tow, but I'm pretty sure that I don't react to every transient "light wing" when off tow. When flying in turbulence, the light wing will quickly alternate from side to side and a left-right PIO in turbulence is not helpful.

But, is flying below the ridge line an asymmetric special case? Should I counter every windward light wing with aileron and NOT likewise counter a light wing on the ridge side? (I'm not looking for flight instruction on the internet, I just want to hear some opinions.)

[[email protected]]

On Saturday, February 1, 2014 2:27:25 PM UTC-8, Ralph Jones wrote:

> 319 AGL at 60 knots.

I haven't done the specific math. At a 45-degree bank you are trying to solve for turn radius=altitude. It will be a different ratio at other bank angles. A shallower bank angle at the same airspeed will cross over at a higher altitude (cross-over is where wing stays on a single point on the ground). Turn radius is a function of bank angle and airspeed so presumably there are a range of altitudes where the visual cues go one way versus the other.

Of course the main point is not to get fooled into over-ruddering your turns at too slow an airspeed down low. Trying to thermal up against a steep mountain has a similar, even more disorienting effect as the "horizon" goes up and down in front of you around the circle. Add to this thermal boundary and ridge shadow shear and/or wind drift effects and you can end up being your own worst enemy.

9B

[[email protected]]

On Saturday, February 1, 2014 5:27:25 PM UTC-5, Ralph Jones wrote:
> On Fri, 31 Jan 2014 11:06:55 -0700, "Dan Marotta" > wrote: [snip] > >Above a certain altitude when you make a coordinated turn with a given bank >angle the inside wingtip traces a circle in the opposite direction vs the >ground. In a low turn to final the wingtip goes the same way around versus >the ground leaving a subtle sensation that the turn lacks enough rudder. > [snip] Pivotal altitude. Equal to TAS in knots, squared, over 11.3, or about 319 AGL at 60 knots. rj

I would be very interested in understanding:
First- what "pivotal altitude" is (definition).
Second- how was equation described developed.

UH

[[email protected]]

On Sunday, February 2, 2014 11:16:30 AM UTC-8, wrote:

>A shallower bank angle at the same airspeed will cross over at a higher altitude

Oops - I meant you get lower altitude crossover at lower bank angles. At 60 knots and 45 degree bank the cross-over is 450'. At 60 knots and 30 degree bank the cross-over is 368 feet and at 60 knots and 15 degree bank it's 330 feet AGL. The absolute minimum for the shallowest possible bank angle is 318 feet AGL.

9B

[[email protected]]

On Sunday, February 2, 2014 12:33:10 PM UTC-8, wrote:

> I would be very interested in understanding:
>
> First- what "pivotal altitude" is (definition).

The pivotal altitude is the altitude at which the line along the wingspan of a turning glider, if extended to the ground, will intersect the ground at the vertical projection of the center of the turning circle of the glider. Below this altitude the line extending out from the wing will intersect the ground at a point between the center of the circle and the vertical projection of the glider on the ground and will appear to trace a circle on the ground that is going in the same direction as the glider is turning (clockwise or counter-clockwise as the case may be). Above the pivotal altitude the extended line along the wingspan will intersect the ground at a point on the opposite side of the center of the circle and will appear to trace a circle on the ground that is anti-clockwise to the turning direction of the glider.

> Second- how was equation described developed.
>
Two equations:
1) Radius of the turn= R=V^2/(32xsin(alpha)), where V is TAS in ft/sec and alpha is the bank angle.

2) Determine pivotal height (H) by setting the ratio of AGL height (H) to R equal to TAN(alpha) so the wingtip points at the turning center on the ground. For a 45 degree bank TAN(alpha)=1 so the radius equals the pivotal height.

It's a subtle peripheral vision effect that can lead the pilot to think they are over-banked or under-ruddered on a low turn to final because the inside wing isn't retreating against the background they way it does up higher.

9B

[[email protected]]

I believe that what really counts in determining the pivotal altitude is groundspeed rather than airspeed. So your equation to determine the pivotal altitude using airspeed applies only if there is no wind. There is a maneuver "eights on pylons" in power planes, where you stay at exactly the pivotal altitude around 2 pylons on the ground during the maneuver. If done correctly, the pivot points or pylons on the ground will appear to remain motionless in relation to the extended centerline of your wingtip. If not, and you are either above or below the pivotal altitude, the pivot point on the ground will appear to be moving, either forward or backward relative to your wingtip, depending on whether you are above or below the pivotal altitude. If there is any wind, it is necessary to actually change the altitude of the aircraft continuously as you go around the pylons in order to stay at the pivotal altitude. This is because your groundspeed changes due to the wind, hence the pivotal altitude constantly changes. The faster the groundspeed, the higher the pivotal altitude. There is a pretty good explanation of it here.:
http://m.aopa.org/asf/publications/inst_reports2.cfm?article=226

[[email protected]]

On Monday, February 3, 2014 10:39:57 AM UTC-8, wrote:
> I believe that what really counts in determining the pivotal altitude is groundspeed rather than airspeed. So your equation to determine the pivotal altitude using airspeed applies only if there is no wind. There is a maneuver "eights on pylons" in power planes, where you stay at exactly the pivotal altitude around 2 pylons on the ground during the maneuver. If done correctly, the pivot points or pylons on the ground will appear to remain motionless in relation to the extended centerline of your wingtip. If not, and you are either above or below the pivotal altitude, the pivot point on the ground will appear to be moving, either forward or backward relative to your wingtip, depending on whether you are above or below the pivotal altitude. If there is any wind, it is necessary to actually change the altitude of the aircraft continuously as you go around the pylons in order to stay at the pivotal altitude. This is because your groundspeed changes due to the wind, hence the pivotal altitude constantly changes. The faster the groundspeed, the higher the pivotal altitude. There is a pretty good explanation of it here.:
>
> http://m.aopa.org/asf/publications/inst_reports2.cfm?article=226

Yes - good point. If you are making a crosswind to upwind turn to final the stiffer the breeze the lower the crossover altitude.

9B